Epigenetic regulators have recently received increasing attention as participants in pathogenic processes in human disease, as well as exciting new drug targets due to their enzymatic activities. A goal of this proposal is to generate an animal model in which a paradigmatic epigenetic regulator, MLL1, can be modulated to study a wide variety of human diseases in which this broadly-expressed protein plays an important role. This animal model utilizes Flp recombinase-directed integration to introduce an epitope-tagged, full-length MLL1 cDNA into embryonic stem cells in which doxycycline addition induces MLL1 cDNA expression. Roles for MLL1 in T-cell memory, neural stem cell specification, hematopoiesis, autism spectrum/cognitive disorders, Weideman- Steiner syndrome, skin and muscle stem cell function, neuroepithelial and hematologic cancers, vascular and craniofacial development have been established through a variety of individual approaches, however, the animal models to study the disease mechanisms have been lacking. In this proposal, we will generate an animal model that can accelerate discoveries in all of these fields. This model will be utilized by the co-PIs to test two major hypotheses. The proto-oncogene MLL1 is disrupted by chromosomal translocations or amplification in several types of leukemia with particularly poor prognosis. In contrast, loss of MLL1 in the hematopoietic system results in loss of self-renewal and proliferation defects in hematopoietic stem cells (HSCs). Based on our preliminary data, we will test the hypothesis that transient induction of MLL1 protein can enhance self-renewal in fetal and/or adult HSCs in a manner that could be clinically useful. Thus, the functional consequences of increased MLL1 levels will be determined using this model, particularly the effect on self- renewal and differentiation of HSCs. The impact on epigenetic modification of MLL1 target genes instrumental in self-renewal will be also be determined. The discovery of pathways that enhance self-renewal in HSCs may be directly relevant to efforts to treat umbilical cord blood or pluripotent sources of hematopoietic cells for transplantation. The second hypothesis is that chronic, sustained expression of wild-type MLL1 protein will lead to a myelodysplastic syndrome or leukemia. Our preliminary data illustrate that this Flp-mediated strategy using MLL1-fusion oncoproteins is feasible and can yield novel insights not possible with traditional retroviral- mediated MLL1-fusion leukemia models. Currently, there are no animal models in which MLL1 expression can be sustained in a manner that reflects gene amplification, thus no pre-clinical model for testing novel therapeutic strategies that may be effective in acute leukemia harboring MLL1 amplifications. Establishing the sufficiency of sustained MLL1 expression in myelodysplastic syndrome/leukemia will set the stage for testing the role of this epigenetic regulator in conjunction with other common mutations that occur in acute leukemia.